Abstract Body (Do not enter title and authors here): Background: Dietary stearic acid (18:0), a saturated fatty acid (SFA) commonly present in Western diets, has an LDL-C lowering effect compared to shorter chain SFAs such as palmitic acid (16:0), and a similar effect compared to oleic acid (18:1). However, the underlying mechanisms remain unclear.

Hypothesis: We tested the hypothesis that the hypocholesterolemic effect of dietary 18:0 and 18:1 relative to 16:0 is modulated by alterations in cholesterol and bile acid (BA) metabolism.

Methods: This secondary analysis used archived plasma and fecal samples from a randomized crossover feeding study (N=20 mildly hypercholesteremic postmenopausal women, 64±7 years, BMI 26.4±3.4kg/m²). Participants consumed each of 3 isocaloric diets enriched in either 18:0, 16:0 or 18:1 for five weeks with a 2-week washout. Primary (P) and secondary (S) BAs, and their conjugates were measured in fecal, fasting and non-fasting (NF) plasma samples using the Biocrates MxP Quant 500 kit and Quadrupole Time-of-Flight mass spectrometry. Fasting and NF plasma cholesterol synthesis (lathosterol) and absorption (-sitosterol) markerswere quantified using gas chromatography. Mixed-effect and generalized linear mixed models were used to test the difference in outcome measures among diets, with Tukey-Kramer post hoc comparison. Spearman correlation coefficients with FDR adjustment was calculated between BA, cholesterol synthesis/absorption markers, and CVD risk factors.

Results: Compared to the 16:0 diet, consumption of the 18:0 diet resulted in significantly lower fasting and NF plasma lathosterol (-22%); higher -sitosterol (19%); higher fecal PBAs (31%) and lower fecal SBAs (-17%) concentrations. Plasma PBAs were significantly lower in the fasted state (-34%), but higher in the NF state (21%; 18:0 vs. 16:0). Interestingly, conjugated PBA and SBA concentrations in the NF state were significantly higher after participants consumed the 18:0 compared to the 18:1 diet (all p <0.05). Plasma NF PBAs were positively associated with -sitosterol (r=0.56, p<0.05), while plasma fasting PBAs were negatively associated with lathosterol (r=-0.58, p <0.05). Conjugated PBA and SBA concentrations were negatively associated with LDL-C, hsCRP, E-selectin and insulin concentrations in the fasted but not NF state.

Conclusion: The favorable effects of 18:0 on CVD risk factors may be modulated, in part, by changes in BA and cholesterol metabolism, with distinct effects in the fasted and NF states.